Such a linear gas burner, described in the European patent No. 0190091, offers great advantages from a security point of view, as the mixing of the gas with the blown air occurs only at the inlet of the combustion zone, and from a convenience point of view as it has a very high ratio between its maximum power and its minimum power. This is why it is very widely used, notably in the food industry and in the textile industry. This is the case in particular with textile finishing machines, especially drying and heatsetting tenters where it permits both an optimal distribution of the heat over the whole surface of the product to be treated and a modulation of power of up to 1:50 which, in uses as different as the drying of the fabrics or the heatsetting of the dyes, permits treatment at very precise temperatures whatever the heat requirement. In this perspective, such linear gas burners are starting to be used in place of steam, of electricity or of thermofluid heating, to inwardly heat drying drums formed by a rotating metallic casing over which passes a band of a product to be dried such as a fabric, a sheet of paper, etc. Firstly, gas is, at present, a cheap form of energy, which can be used with a very high output and secondly, it enables higher temperatures to be obtained than with steam heating, which is necessary in certain heat treatments. Moreover, more precise and more evenly distributed temperatures are obtained, notably compared with thermofluid or electric heating. Finally, a gas-burner is a heating device which can be used independantly without being linked to other installations, and in particular to a boiler room. Moreover, compared with other types of linear gas-burners such as pre-mixing burners, the use of a burner of the type indicated above is particularly worthwhile due to its great possibility of power modulation, which allows a very precise adjustment.
However, the need exists to be able to adjust in quite a precise way, the width of the zone to be heated. For example, in a fabric drying drum, trials have shown that the evenness of the drying is closely linked to the active width of the gas-burner. If the fabric is of a greater width than the burner, its selvages will be insufficiently dried. If the burner has an active width greater than that of the fabric, the latter will be excessively dried near its selvages. Indeed, the zones at the extremities of the drum, which are not covered by the fabric but are nonetheless heated by the burner, can reach an excessive temperature of up to around 400.degree. C. which is detrimental both for the drum and the fabric. If burners which pre-mix the gas and the combustion air are used, it is not too difficult to conceive a mechanism which reduces the active length of the burner, by progressively sealing the gas outlet apertures. Publication GB-A-204 756 shows a linear pre-mixed gas burner which has a distributor tube supplied half way along its length and sealed by two pistons fitted on an axial shaft threaded in two opposite directions. However, this type of pre-mixing burner belongs to a technology which is fundamentally different from that of pure gas burners and has other drawbacks mentioned above. In a pure gas and blown air burner of this type, one solution could consist in dividing the burner up into several linear or punctual burners, with outside valves on the corresponding gas supplies. However, such a solution is complicated, bulky and costly, and it only allows a relatively rough adjustment thread.